Printed circuit boards
PCB’S as these components are commonly known as, are the brains of a central heating boiler.
The PCB is what makes all electronic components in your boiler interconnect, allowing your boiler to work efficiently. As an example, before the boiler ignites, it sends a signal for the fan to start. It wants to see an air pressure change from the air pressure switch, to confirm the fan is running, before igniting.
These impressive components of boilers really are a fantastically engineered part.